It is and has been common practice in the engineering field to construct large electronic systems from a plurality of printed circuit (PC) boards or PCBs that are arranged and installed in one or more card cages or chassis. The card cages are typically constructed from metal or some other sturdy material and serve to mechanically support the multitude of electrical PC boards making up the apparatus. Each card in the cage plugs into a backplane which may be either passive or active.
The card cages typically contain a variable number of PC boards or cards, i.e., electronic and electrical modules, generally referenced modules. The card cages are designed to provide a plurality of functions to the modules contained therein. As stated above, the major function provided is mechanical support for all the cards to be placed in the cage. Also, the cage serves to provide a mounting means for the backplane which ties all the cards together. The cage also provides means for the mounting of the power supply which may consist of more than one unit. In addition, the cage provides the means for management of configuration and functional parameters support. The cage further supports any other service or functional support required by the PCBs enclosed therein. Card cages or boxes that provide one or more services to the PC boards are also termed platforms, as used in the description hereinbelow.
In the typical scenario, platforms for a multiple number of modules normally generate large quantities of heat that must be removed. The most common method for the removal of this heat is forced air ventilation by means of one or more electrical fans. The use of forced air ventilation has, however, a number of disadvantages and drawbacks that must be considered when the removal of heat is accomplished by simply installing fans in the platform.
First, the number of modules installed in the platform may vary between a minimum of none to the maximum number available in the given particular platform. The air flow path can change in accordance with the number of modules installed in the platform. In many cases, the air flow path can change in such a way that one or more modules present in the platform receive a minimal of cooling while most of the air flow passes unused through the spaces or slots unoccupied by modules. In such cases, the designers of the platforms usually increase the airflow by a significant amount in order to provide sufficient cooling to make the worst case scenario of possible combinations of present and absent modules still acceptable.
This prior art approach suffers from several disadvantages as described below. First, increasing the airflow requires additional fans and each fan must have increased capacity. It is not guaranteed, however, that each platform will have sufficient room to accommodate the larger number of fans required by this approach.
Second, the increased number of fans generates additional noise. In most countries, platforms are required to comply with government regulations and/or standards limiting noise production. The use of powerful fans, in large numbers, makes compliance with statutory or standards based noise limitations more difficult if not practically impossible.
Third, high capacity fans typically operate under increased stress. It is thus highly probable that these larger capacity fans will have lower Mean Time Between Failure (MTBF) ratings.
Fourth, additional fans, especially higher airflow capacity fans, require significant additional power from the electrical power plant in the platform.
An illustrative example of this problem will now be presented. Take, for example, a 16 slot card chassis that can host as few as a single module or as many as 16 modules. In cases when most of the space inside the chassis is not occupied by modules, the airflow may pass through the free space leaving a single module almost or completely without cooling airflow. In particular, consider a platform, i.e., chassis, with eight modules placed on one side of the platform. In such an arrangement, the airflow is severely reduced within the area populated by modules, which is the area in need of cooling. Most of the airflow takes the path of least resistance, which is the unpopulated portion of the platform. In order to provide sufficient cooling to the modules, the designer of the platform of this example, must place six fans, each providing 50 cfm. In the case when all 16 modules are inserted in the platform, only 30 cfm per fan is sufficient to provide adequate cooling.
Thus, there is a strong felt need for a platform (card chassis) that consumes less power, generates less noise, does not lower the MTBF of the fans installed therein and which is able to supply cooling airflow to the modules (PCBs) that are installed therein regardless of the number of modules installed and their position in the platform.